Rep Power: 528

I've heard people say things like "this part decreases torque but gives more horsepower." How is that possible? If HP = ((TQ * RPM))/5252), then any time you have less TQ you're going to have less hp, too. See?

If you're making 100ft/lb of torque at 2000rpm that's 38hp
If you're making 100ft/lb of torque at 4000rpm that's 76hp
but if you're only making 90ft/lb of torque at 2000rpm that's 34hp. Less torque = hess hp.

 
Rep Power: 282

I think what you mean is high end horsepower vs low end torque. A huge straight exhaust for example will probably give you more high end horsepower but less low end torque. Is this what you mean? If so, in this case it has to do with backpressure. Read this,
http://web.tampabay.rr.com/redroby/3liter/exhaust.html

 
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ditto, back pressure is the key, i know where youre comin from with the calculation, but this is one of those exceptions i guess, i mean the tq losses are dyno proven

 
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Back pressure only comes into play on a 2 stroke engine. Parts don't change the amount of tq. (Well they do but it's linear) what gets changed is the curve. I'm lazy so I'm just going to link to more info.

1
2

and of course the Back pressure myth

 
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Thanks for the info. Reading links now...

 
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intresting..

 
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2-strokes are more depedent on backpressure but 4-strokes are still as well, at least for low-end torque. Someone do some more research and posts links please, I'm at work...

 
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i believe it could also be from tuning timing and or other engine componets

if you look at a dyno graph
here's one right here

torque decreases after a certain point, where hp and torque crosses, i guess if the power band curve is changed, the torque band would drop earlier and the hp band would increase earlier too

 
Rep Power: 528

Torque and HP are always supposed to cross at 5252 rpm based on the formula. So I'm always weary of dynos that don't show this. They do cross at 5252 in the dyno chart above.

Does anyone know why they don't cross at 5252 sometimes? Is it because the dyno is BS, or is it because there's some factor that could cause them to not cross at 5252?

 
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Can't help you with that but have you figured out backpressure yet?

 
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cars require back pressure because it uses vaccume and pressure for the cylinders to work, if you have 0 back pressure, a/f mixture would just go from the intake valve directly to the exhaust valve, some engine also wants to keep some of the exhaust gas to keep the cylinder cooler, thus, can run more advances on timing

 
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Yes, but he inquired as to how a part could cause a loss of torque, I pointed him in the direction of changing backpressure = changing low-end torque.

 
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4 strokes deffinately need backpressure. backpressure prevents the free flow of exhaust gases from the engine. free flowing exhaust means free flowing intake right out of the ehxuast hehe. pull off your header and run like that, see how much torque you lose.

 
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Losing backpressure causes you to lose low end torque..... and yes you do lose hp as a result..... in the low end. However, you gain torque and hp in the high end. Hp is not equal throughout the powerband.... so when people say a mod makes you lose torque and gain hp they probably don't realize it, but it means you are losing low end torque and hp, but your MAX HP is increased. Yes, you do gain maximum hp as yer max hp is in the high end. No mod linearly (is that a word? lol) adds hp and torque throughout the powerband.

 
Rep Power: 528

I read an article called "the Myth of Backpressure" but I forgot what it was about and where it was.

Apparently BMW's have something that automatically adjusts some things to get the optimal amount of backpressure at various rpms, or something like that.

 
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It wouldn't go straight out the exhaust valve. the exhaust valve is closed. They have springs to keep them closed (called valve springs..go figure) The are opened by cams (one cam on our car..two on others... hence SOHC and DOHC)The vacum you speak of is caused when the piston goes down and causes negative pressure in the cylinder. Since air cant come in throught the exhaust (the valve is closed remember?) it gets sucked in through the intake valve. For the record drag cars use open headers IE NO back pressure. Using exhaust gas is more for emmisions than cooling. Your just burning the unburned fuel in the mixture.

 
Rep Power: 528

I think I understand it now.

If you lose x amount of torque in lower rpms but gain that same x amount of torque in the upper end, you will have a net gain of hp because for any given amount of torque, higher rpm's = more hp.

100 ft/lbs @ 2000rpm = 38 hp
100 ft/lbs @ 6000rpm = 114.2 hp

That makes sense now.

 
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we dont need to lose anymore torque because even if get it in high end we still wouldnt go fast enough. its not worth the trade.

 
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It depends on whether the torque and hp scales are the same. If they are, it crosses at 5252 as you indicated. If the operator scales them differently, they won't cross at 5252. The way to check for BS on a dyno chart that doesn't scale them the same is to multiply torque at any rpm by rpm and divide by 5252. That should give you the hp reading.

The charts I got this weekend were not scaled the same. We scaled them differently to help us see what was happening.

 
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Thanks. Yeah after I posted that question I realized that if the scales were different the numbers wouldn't necessarily cross at 5252 but they should still be the same number.

Thanks.

 
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Hey here's another question.

Since we've already established that:

100 ft/lbs @ 2000rpm = 38 hp
100 ft/lbs @ 6000rpm = 114.2 hp

Would a car making 100lb/ft @ 6000rpm be pulling harder than if it were making 100lb/ft @ 2000rpm? The hp is higher at 6000rpm, but isn't a car's accleration based on its torque? Cuz the hardest pull is at whatever rpm the peak torque is.

 
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You didn't read the link I posted did you

 
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Yes Acceleration is based on Torque, in your example above, the car making 100lb/ft @ 2000 rpms would accelerate quicker then the other car up to 2000RPMs, after that the torque curve falls where the other car is still making more. HorsePower is just an expression of Torque at specific RPM's...... Thats only usefull in telling you how a car might perform, What you need to look at is Torque @ RPM's, You want good torque at a High RPM!

Examples,
car 1 325 hp @ 3,300 rpm and 560 lb-ft @ 2,000
car 2 350 hp @ 5,200 rpm and 375 lb-ft @ 4,400
which one of these cars based on just these numbers would run a 1/4 mile quickest?
both are making about the same HP, but one is making MUCH more Torque then the other, but at a much lower RPM...........




















Car 1 is a chevy Corvette
Car 2 is a Ford F350 Power Stroke V-8.